Compositions comprising a polyphenylene ether resin and a styrene resin are well known as useful engineering thermoplastics, for molding, extrusion and the like. They are described in Cizek, U.S. Pat. No. 3,383,435, which is incorporated herein by reference.
Such compositions are normally flammable, particularly if high proportions of styrene resin are present, and aromatic phosphate compounds, e.g., triphenyl phosphate, are used to retard or eliminate flammability. Haaf, U.S. Pat. No. 3,639,506, also incorporated herein by reference, discloses that triphenyl phosphate has a tendency to reduce physical properties and describes the use of combinations of aromatic phosphates and aromatic halogen compounds to flame retard the composition, without markedly lowering resistance to distortion by heat.
Dialkyl pentaerythritol diphosphonates have been reported in Friedman, U.S. Pat. No. 3,141,032, to be generally superior as plasticizers and as flame retardants for synthetic resins. However, it is disclosed that from 10 to 60 parts of the said phosphonate per 100 parts of resin (9.1-37.5 parts per 100 parts of the conbination, by weight) is the proper amount to use, with a stated preference of 30 parts per 100 parts of resin.
It has now been discovered that cyclic phosphonate compounds by themselves are effective nonplasticizing flame retardant additives for the compositions of polyphenylene ethers and styrene resins, at an unexpectedly lower concentration. Moreover, such cyclic phosphonate materials are just as effective in flame retardance in this system is triphenyl phosphate and provide compositions with substantially the same impact resistance as the polymer composition itself.